This invention relates generally to the field of refractive corneal surgery and, in particular, to the use of a refractive laser in conjunction with an automated lamellar keratoplasty, so-called LASIK (laser-in-situ keratoplasty). The present invention teaches a novel apparatus for improving the creation of the lamellar corneal flap.
LASIK (laser-in-situ keratoplasty) is a refractive laser procedure used to correct refractive errors such as myopia, hyperopia, and astigmatism. Aspects of the procedure are disclosed and claimed in U.S. Pat. No. 4,840,175, to Peyman et al and No. 4,903,695, to Warner et al. In general the refractive procedure requires the creation of a lamellar corneal flap. A variety of mechanical cutting devices have been described to create the lamellar flap. These devices, generally known as "microkeratomes", differ principally in how they drive the blade (e.g. gears vs. cable) and how they flatten (applanate) the cornea during the lamellar cut. For example, U.S. Pat. No. 5,586,980, to Kremer et al discloses a microkeratome device.
The prior art microkeratome devices all share in common the creation of flaps of variable depth and size, depending somewhat upon corneal curvature. That is to say, "steep" corneas (e.g. central keratometry greater than 46 diopters) produce flaps of different size and thickness than do "flat" corneas (e.g. central keratometry less than 41 diopters). This variation occurs because the prior art devices all require an applanation of the cornea through a suction ring during the keratectomy. In the case of an extremely flat cornea, for example, the likelihood of an amputated flap (corneal cap) is very high. Variations in flap diameters compromise the laser correction by limiting the diameter of the ablation, which in turn affects centration and the degree of maximum correction and also may result in side effects, such as halos and ghosting.
Moreover, all these prior art devices use high suction pressure (elevates intraocular pressure to 90 mm Hg or more) to firm up the globe during the keratectomy. Loss of suction during the keratectomy can result in complications such as irregularly thick flap, lacerated flap, amputated flap, and aborted flap creation. Generally speaking, if the suction break occurs before completion of the keratectomy and it is detected by the surgeon, the case is aborted. The high intraocular pressure (often 90 mm Hg or more) created by a suction apparatus of the prior art can have well-recognized, serious complications such as intraocular vascular occlusion, macular hemorrhage, and retinal detachments.
Prior art lamellar keratectomy microkeratomes also share another common limitation, i.e., they all create a dome-shaped flap. The central thickness is generally desired to be 150-200 microns in order to provide adequate peripheral thickness. This flap contour is disadvantageous because it can mask the underlying stromal contour created by the laser ablations. In a myopic correction, for example, the laser creates a central concavity in the stromal bed, but the flap has a central convexity because of its dome shape.
Furthermore, the thin flap edges created by prior art devices are prone to melt. The flap edges are on the surface or the cornea, resulting in a greater likelihood that the flap will be distorted, displaced, or torn free by eyelid blinking or minor trauma such as rubbing the eyes. Prior art procedures rely upon hydrostatic pressure to seal the flap to the surface of the cornea.
Besides mechanical cutting microkeratomes which use oscillating metal or gem blades, other cutting modalities have been introduced such as pressured water jets, as provided in U.S. Pat. No. 5,643,299 to Blair, or lasers such as erbium:YAG. Nevertheless, these prior art techniques and devices share the aforementioned limitations of the mechanical cutting microkeratomes.
In my co-pending application Ser. No. 09/188,160, I provide an apparatus and method to eliminate the aforementioned limitations of the prior art. As such, that device provides an optionally thinner, but uniformly thick corneal flap of precisely determined diameter, regardless of corneal curvature. Furthermore, that device more reliably reproduces the curvature created by the laser, eliminates the risk of flap amputation (free cap), eliminates the risk of flap distortion, displacement, or tearing, and reduces the risk of epithelial ingrowth and eliminates the risk of complications associated with high suction pressure, such as intraocular vascular occlusion. However, the present invention discloses an improvement over that device and provides a novel apparatus for the microkeratome device used in creating a lamellar corneal flap and is intended to be used so as improve the method of performing the procedure disclosed in co-pending application Ser. No. 09/188,160.